Methods for washing and protecting photographic silver images



United States Patent METHODS FOR WASHING AND PROTECTING PHOTOGRAPH-[1C SILVER IMAGES Edwin H. Land, Cambridge, Mass., assignor to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware No Drawing. Application February 24, 1954 Serial No. 412,394

7 Claims. (Cl. 96-29) This invention relates to protective environments for photographic silver images and, more particularly, to methods for and products of protecting photographic silver images after their formation with compositions containing gelatin.

Objects of the invention are: to provide a novel process for protecting a photographic silver image after its formation by intimately associating its silver grains with gelatin; to employ hydrolyzed gelatin in such a process; and to provide, as novel products, protected photographic silver images produced by such a process.

Other objects of the invention are: to provide, in a novel process for washing and protecting an extremely thin photographic silver image, the step of applying thereto, in a layer, a composition containing an aqueous solution of gelatin and the step of drying the layer to produce a residue composed primarily of gelatin; to employ in such a process an aqueous gelatin dispersion which remains fluid when stored for a prolonged period throughout a wide temperature range and which gels rapidly under ambient conditions when spread in a thin layer; to employ in such a process a composition containing an aqueous solution of hydrolyzed gelatin; and to provide, as novel products, protected photographic silver images produced by a process.

Further objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the product possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the appended claims.

The processes of the invention are particularlyapplicable to extremely thin photographic silver images of the type which may be produced by applying a processing composition containing a silver halide developer and a silver halide solvent to a photoexposed photosensitive silver halide sheet and an image-receptive sheet which are in superposed relation. The composition acts to reduce exposed silver halide to silver, to react with unreduced silver halide to form a water-soluble, complex Patented Oct. 7, 1958 gelatino silver halide stratum of a photosensitive sheet and the silver-receptive stratum of an image-receptive sheet, for example, by advancing the sheets between a pair of pressure-applying rollers. The elements are maintained in superposed relation for a predetermined period, ordinarily of approximately 40 to 120 seconds in duration, during which exposed silver halide is reduced to silver and unreduced silver halide forms a watersoluble, complex silver salt which diffuses through the layer of composition to the image-receptive stratum, where, upon being reduced to silver, it forms a visible print. At the end of this period, the photosensitive sheet, preferably together with the layer of composition, is stripped from the image-receptive sheet.

An image-receptive stratum of the foregoing type in one form includes silver precipitating nuclei dispersed in a macroscopically continuous vehicle comprising submacroscopic agglomerates of minute particles of a waterinsoluble, inorganic, preferably siliceous, material such as silica aerogel. Silver grains precipitated in the foregoing manner are concentrated primarily at the surface of this stratum. This stratum, both before and after receiving these precipitated silver grains is extremely thin, preferably being approximately 1 to 8 microns thick. Materials of the foregoing type are specifically described in copending U. S. application Serial No. 727,385, filed by Edwin H. Land on February 8, 1947, for Photographic Product and Process, Patent No. 2,698,245, and Serial No. 164,908, filed by Edwin H. Land on May 29, 1950, for Photographic Silver Halide Product and Process, now abandoned.

Preferably, there is interposed between the imagereceptive stratum and its support a water-impermeable layer capable of preventing the penetration of moisture from the processing liquid into the support. This layer is responsible for the production of av substantially dry image only shortly after the photosensitive layer is stripped from it. The water-impermeable layer, for example, may be composed of-unplasticized polymethacrylic acid or one of the cellulosic esters such as cellulose I nitrate, cellulose acetate, cellulose butyrate, cellulose type are described in detail in Patent No. 2,543,181, issued propionate, cellulose acetate butyrate, or cellulose acetate propionate. Preferred, however, are such rubbery polymers as polyvinyl butyral. If the support is water impermeable, of course, a discrete, water-impermeable layer need not be provided.

An extremely thin, photographic silver image of the foregoing type ordinarily retains traces of the photographic reagents With which it has been processed and the continued presence of which may adversely affect its stability. For example, silver may be oxidized by sulfur from the residue of sodium thiosulfate which has been employed as a solvent. Also, silver may be oxidized by such agents as hydrogen sulfide often present in the atmosphere. Furthermore, traces of unexposed developer, if oxidized by atmospheric oxygen, may slightly discolor the highlights of the image.

It has been proposed, in order to improve the stability of such an image, to coat it with a composition comprising an aqueous dispersion of a film-forming material. The water acts to wash traces of the photographic reagent from the print and to so distribute the film-forming material that it provides the image with a protective coating when the dispersion is dried. Processes of the foregoing type are more fully described in application Serial No. 302,746, filed in the name of Edwin H. Land on August 5, 1952, for Method of Improving the Stabilization of Finished Photographic Prints by Applying a Liquid Composition Thereto, and the Product of Said Method.

In accordance with the present invention, the stability of such an image is improved by coating it with a composition comprising an aqueous dispersion of gelatin. By

virtue of the fact that the silver of this image is concentrated at the surface of an extremely thin, silver-receptive stratum, it is immediately accessible to gelatin from an aqueous dispersion so applied. When this dispersion is dried, the gelatin is contiguous or otherwise intimately associated with the silver. The gelatin, which under these conditions appears to give the silver more than mere mechanical protection, provides the image as a whole with a protective environment of unexpected excellence.

Preferably, this dispersion is so constituted as to remain fluid when stored for a prolonged period throughout a wide temperature range and to gel rapidly after being spread in a thin layer when dried under ambient conditions. Where the dispersion is not to be applied immediately after its formulation, its premature gelling may be prevented, for example, by the addition of an antigelling agent such as an amide or by the use of a hydrolyzed gelatin of low molecular weight.

Preferably, the composition is applied to one face of the image by means of an absorbent applicator composed, for example, of flannel, cotton batting or cellulose sponge charged with the composition. When the face of the image is swabbed with such an applicator, residual reagents of the image are dissolved and, for the most part, transferred into the applicator, and the image becomes coated with a thin layer of the dispersion. The dispersion can then be dried to form a protective coating.

In one form, a dispersion for washing and protecting an image of the contemplated type contains a to 40% aqueous solution of a class to 30 Bloom gelatin to which has been added an amide such as formamide or urea in sufiicient quantity to reduce the gel point of the solution to a suitable temperature. A typical formulation of such a solution follows:

Example 1 To 100 cc. of a 30% aqueous solution of 30 Bloom tin having polypeptide chains on the average of approximately 7 amino acid units.

The solutions of Examples 1 and 2, after being swabbed by means of an absorbent applicator onto a thin photographic silver image of the above-described type, rapidly dried under ordinary atmospheric conditions to form a coating having excellent protective properties for the image.

Since certain changes may be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of forming a stable photographic print, said method comprising the steps of spreading, between a gelatin-free, silver-receptive stratum and an exposed gelatino silver halide stratum, in a layer, an aqueous solution of a silver halide developer and a silver halide solvent whereby exposed silver halide is reduced to silver and unreduced silver halide forms a water-soluble complex silver salt which diffuses through the layer of solution to the image-receptive stratum Where, upon being reduced to silver, it forms a visible print, stripping said print from said gelatino silver halide stratum, said print containing a residue of said solution, swabbing said print with an absorbent applicator charged with a composition containing an aqueous dispersion of gelatin in order to wash said residue from said print into said applicator, and drying composition retained on said print in order to intimately associate said gelatin with said silver in said print.

gelatin was added /2 g. of methyl paracept as a preservative, 3 g. of valeramide and 3 g. of caproamide as drying agents, 5 cc. of formamide as an antigelling agent, 5 cc. of a 25% aqueous solution of saponin as a spreading agent and 10 to 15 cc. of water as diluent.

It has been found that a solution of hydrolyzed gelatin, fluid enough at ordinary temperatures to be readily spread upon an image of the contemplated type, when dried, provides the image with a hydrolyzed gelatin environment possessing protective properties for silver similar to those of ordinary gelatin. Preferably, such a hydrolyzed gelatin is not less than 15% and not more than by total weight of the solution and has polypeptide chains on the average of not less than 4 and not more than 12 amino acid units. Such a solution remains fluid throughout a wide temperature range and dries within a short period after it is applied.

In general, such a solution may be prepared by hydrolyzing either alkali or acid extracted gelatin with a proteolytic enzyme such as papain, pepsin or trypsin and, when the desired degree of hydrolysis is reached, by stabilizing the solution with a preservative such as methyl p-hydroxy benzoate, cetyl-pyridinium chloride or oxyquinoline. A typical formulation of such a solution follows:

Example 2 To a dispersion of 44 pounds of 100 Bloom, alkali pretreated gelatin in 19 gallons of water was added a dispersion of one-half pound of papain in one quart of water. The resulting mixture at 60 C. was stirred for a period of from 12 to 15 minutes during which hydrolysis of the gelatin occurred. At the end of this period, in order to halt the hydrolysis, one quart of 7% hydrogen peroxide was added and the temperature raised as rapidly as possible to 95 C. After the resulting solution was filtered and cooled, one-half pound of methyl paracept was added as a preservative. The solution then contained, by total weight, approximately 22% of hydrolyzed gela- 2. The photographic process of claim 1 wherein said silver-receptive stratum is siliceous and is 1 to 8 microns thick.

3. The photographic process of claim 2 wherein said solution contains from 15 to 40% of gelatin.

4. The photographic process of claim 3 wherein said gelatin has a rating of from 20 to 30 Bloom.

5. A method of forming a stable photographic print, said method comprising the steps of applying in a thin layer to a gelatin-free, image-receptive stratum and an exposed gelatino silver halide stratum an aqueous solution of a silver halide developer and a silver halide solvent whereby exposed silver halide is reduced to silver and unreduced silver halide forms a water-soluble complex silver salt which diffuses through the layer of solution to the image-receptive stratum where, upon being reduced to silver, it forms a visible print, stripping said print from said gelatino silver halide stratum, said print containing a residue of said solution, swabbing said print with an absorbent applicator charged with a composition containing an aqueous dispersion of gelatin in order to wash said residue from said print into said applicator, and drying composition retained on said print in order to intimately associate said gelatin with said silver in said print.

6. The method of claim 5 wherein said gelatin-free stratum is siliceous.

7. The method of claim 5 wherein said dispersion contains a 15 to 40% aqueous solution of gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 1,543,301 Stock June 23, 1925 2,173,480 Jung Sept. 19, 1939 2,578,282 Bliss Dec. 11, 1951 OTHER REFERENCES 

5. A METHOD OF FORMING A STABLE PHOTOGRAPHIC PRINT, SAID METHOD COMPRISING THE STEPS OF APPLYING IN A THIN LAYER TO A GELATIN-FREE, IMAGE-RECEPTIVE STRATUM AND AN EXPOSED GELATINO SILVER HALIDE STRATUM AN AQUEOUS SOLUTION OF A SILVER HALIDE DEVELOPER AND A SILVER HALIDE SOLVENT WHEREBY EXPOSED SILVER HALIDE IS REDUCED TO SILVER AND UNREDUCED SILVER HALIDE FORMS A WATER-SOLUBLE COMPLEX SILVER SALT WHICH DIFFUSES THROUGH THE LAYER OF SOLUTION TO THE IMAGE-RECEPTIVE STRATUM WHERE, UPON BEING REDUCED SAID GELATINO SILVER HALIDE STRATUM, SAID PRINT CONTAINING A RESIDUE OF SAID SOLUTION, SWABBING SAID PRINT WITH AN ABSORBENT APPLICATOR CHARGED WITH A COMPOSITION CONTAINING AN AQUEOUS DISPERSION OF GELATIN IN ORDER TO WASH SAID RESIDUE FROM SAID PRINT INTO SAID APPLICATOR, AND DRYING COMPOSITION RETAINED ON SAID PRINT IN ORDER TO INTIMATELY ASSOCIATE SAID GELATIN WITH SAID SILVER IN SAID PRINT. 